Project Summary With an estimated 15 to 20% of workers in North America and Europe engaged in nightshift work, the possibility that working at night can result in an increased risk of developing cancer is an important public health concern. Given the discrepancy between the strength of experimental and epidemiologic evidence for the carcinogenicity of nightshift work, studies of biomarkers of effect among nightshift workers can provide important new information regarding the possible etiologic link. The core circadian genes are essential for regulation of circadian rhythms and these genes are involved in a number of mechanisms important to carcinogenesis. Thus, differential expression of these genes, through modulation of DNA methylation, may be a mechanism by which nightshift workers are at an increased risk of developing cancer. The overall objective of this feasibility study is to determine whether differentially methylated sites can be detected in circadian genes between night and dayshift workers using existing samples from a carefully evaluated group of shift workers. This will be the first study with the capability to evaluate such differences in both men and women. The central hypothesis is that circadian disruption associated with working the nightshift results in differential expression of circadian genes that is manifest through differential methylation of loci in these genes. In addition, measures of adaptability to nightshift work, such as chronotype and sleep quality, will be evaluated for a potential impact on DNA methylation among nightshift workers. Using existing blood samples and questionnaire data, the study proposes to evaluate subjects who, at the time of blood draw, were actively working the nightshift for e1 year (n=230) and subjects who were actively working the dayshift for e1 year (n=114). The hypotheses will be tested by pursuing the following specific aims: 1) Evaluate differences in methylation at 334 loci in the 12 core circadian genes between established night and dayshift workers; 2) In exploratory analyses, evaluate if gender, race, chronotype and sleep quality affect the impact of nightshift work on DNA methylation at the 334 loci. This study will provide essential preliminary data for the pursuit of large scale investigations of epigenetic and other biomarkers among shift workers. Furthermore, the results of this study will be useful to the design of future epidemiologic studies examining cancer risk by providing data regarding the collection of the most relevant work history and sleep-related data, the incorporation of potentially useful biomarkers and the generation of new hypotheses. It is anticipated that this line of research will ultimately provide targets for interventions to mitigate negative health effects among nightshift workers.